Method and Acidizing Tool for Deep Acid Stimulation Using Ultrasound

ABSTRACT

A method of deep acid stimulation for a zone to be treated in an underground formation using an acidizing tool, the method including the steps of introducing the acidizing tool into the well bore, introducing the acid formulation onto the well bore wall at the treatment zone and introducing ultrasound energy into the underground formation at the treatment zone. The subsequent acid penetration depth is deeper than the initial acid penetration depth. A method of stress fracturing a portion of an underground formation includes the steps of introducing the acidizing tool into a well bore and introducing the acid formulation and the ultrasound energy at the focused treatment point. The weakened acidized spots in combination with the stress on the underground formation causes oriented stress-induced fractures to form that are fluidly coupled with the well bore. An acidizing tool includes an acid delivery system and an ultrasonic transmitter.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority from U.S. Provisional Application No.61/568,279, filed Dec. 8, 2011. For purposes of United States patentpractice, this application incorporates the contents of the ProvisionalApplication by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The field of invention relates to a method and device for improving theeffectiveness of a matrix acidizing technique by increasing the depth ofpenetration of the acid throughout a subterranean carbonate formation.

BACKGROUND OF THE INVENTION

It is a common practice to acidize subterranean formations in order toincrease the permeability thereof. For example, in the petroleumindustry, acidizing fluid can be injected into a well in order toincrease the permeability of a surrounding hydrocarbon bearingformation, thereby facilitating the flow of hydrocarbonaceous fluidsinto the well from the formation. Such acidizing techniques may becarried out as “matrix acidizing” procedures or as “acid-fracturing”procedures.

In acid fracturing, the acidizing fluid is disposed within the wellunder sufficient pressure to cause fractures to form within theformation. An increase in permeability; therefore, is effected by thefractures formed, as well as by the chemical reaction of the acid withinthe formation.

In matrix acidizing, the acidizing fluid is passed into the formationfrom the well at a pressure below the fracturing pressure of theformation. In this case, the permeability increase is caused primarilyby the chemical reaction of the acid within the formation with little orno permeability increase being due to mechanical disruptions within theformation as in fracturing.

In most cases, acidizing procedures are carried out in calcareousformations such as dolomites, limestones, dolomitic sandstones, and thelike. However, a common difficulty encountered in acidizing these typesof formations is presented by the rapid reaction rate of the acidizingfluid with those portions of the formation with which it first comesinto contact. This is particularly serious in matrix acidizingprocedures. As the acidizing fluid is forced from the well into theformation, the acid reacts rapidly with the calcareous materialimmediately adjacent to the well. Thus, the acid becomes spent before itcan penetrate a significant distance into the formation. For example, inmatrix acidizing of a limestone formation, it is common to achievemaximum penetration with a live acid to a depth of only a few inches toa foot from the face of the wellbore. This, of course, severely limitsthe increase in productivity or injectivity of the well.

Various methods have been attempted to reduce the reaction rate of theacid with the rock formation. For example, others have tried addingreaction inhibitors to the acid formulation. Additionally, other workhas focused on ways to reduce the local temperature in order to slowdown the reaction rate. However, all of these types of solutions sufferserious drawbacks by increasing the cost and complexity of the matrixacidizing operation. Therefore, it would be advantageous to have amethod and a device that provided for an improved deep acid stimulationover those known heretofore.

SUMMARY OF INVENTION

The methods and device provides for matrix acidizing aimed at reachingdeeper stimulation zones in the underground formation. The method usesultrasound energy to push the stimulating acid deeper into theunderground formation.

A method for performing a deep acid stimulation of a zone to be treatedin an underground formation utilizes an acidizing tool. The methodincludes the step of introducing the acidizing tool into a well bore.The well bore is operable to permit access to the underground formation.The well bore is also defined by a well bore wall. The acidizing tool isoperable to introduce an acid formulation onto the well bore wall. Theacidizing tool is also operable to introduce ultrasound energy into theunderground formation. The method includes the step of introducing theacid formulation onto the well bore wall at the treatment zone. The acidformulation includes an acid. The introduction of the acid formulationis such that the acid diffuses into the underground formation at thetreatment zone to an initial acid penetration depth. The method includesthe step of introducing ultrasound energy into the underground formationat the treatment zone. The acid diffuses into the underground formationat the treatment zone to a subsequent acid penetration depth. Thesubsequent acid penetration depth is deeper into the undergroundformation than the initial acid penetration depth.

A method of stress fracturing a portion of an underground formationincludes the step of introducing the acidizing tool into a well boresuch that it is positioned proximate to a focused treatment point. Thefocused treatment point is associated with a portion of the undergroundformation under stress. The acidizing tool is operable to direct theacid formulation and the ultrasound energy at the focused treatmentpoint. The method includes the step of introducing at the same time theacid formulation and the ultrasound energy at the focused treatmentpoint. The simultaneous introduction diffuses acid from the acidformulation into the portion of the underground formation under stress.The acid formulation is introduced at a pressure less than the fracturegradient pressure stressed underground formation. The diffused acidcreates weakened acidized spots in the underground formation understress. The weakened acidized spots in combination with the stress onthe underground formation causes oriented stress-induced fractures toform that are fluidly coupled with the well bore.

An acidizing tool for use in a well bore traversing through anunderground formation includes an acid delivery system operable tointroduce an acid formulation onto a well bore wall of the well bore.The acidizing tool also includes an ultrasonic transmitter operable tointroduce ultrasound energy into the underground formation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention are better understood with regard to the following DetailedDescription of the Preferred Embodiments, appended Claims, andaccompanying Figures, where:

FIGS. 1A-C show an embodiment of the method of using an embodiment ofthe acidizing tool in a cross-sectional view of a pre-formed well bore;

FIG. 2 show an embodiment of the method of using an embodiment of theacidizing tool in a cross-sectional view of a pre-formed well bore;

FIG. 3 shows a cross-sectional view of an embodiment of the acidizingtool;

FIG. 4 shows a cross-sectional view of an embodiment of the acidizingtool;

FIG. 5 shows the histogram depth analysis for both before and after acidformulation exposure on a first core plug; and

FIG. 6 shows the histogram depth analysis for both before and after acidformulation and ultrasound energy exposure on a second core plug.

In the accompanying Figures, similar components or features, or both,may have the same or a similar reference label.

DETAILED DESCRIPTION

The Specification, which includes the Summary of Invention, BriefDescription of the Drawings and the Detailed Description of thePreferred Embodiments, and the appended Claims refer to particularfeatures (including process or method steps) of the invention. Those ofskill in the art understand that the invention includes all possiblecombinations and uses of particular features described in theSpecification. Those of skill in the art understand that the inventionis not limited to or by the description of embodiments given in theSpecification. The inventive subject matter is not restricted exceptonly in the spirit of the Specification and appended Claims.

Those of skill in the art also understand that the terminology used fordescribing particular embodiments does not limit the scope or breadth ofthe invention. In interpreting the Specification and appended Claims,all terms should be interpreted in the broadest possible mannerconsistent with the context of each term. All technical and scientificterms used in the Specification and appended Claims have the samemeaning as commonly understood by one of ordinary skill in the art towhich this invention belongs unless defined otherwise.

As used in the Specification and appended Claims, the singular forms“a”, “an”, and “the” include plural references unless the contextclearly indicates otherwise. The verb “comprises” and its conjugatedforms should be interpreted as referring to elements, components orsteps in a non-exclusive manner. The referenced elements, components orsteps may be present, utilized or combined with other elements,components or steps not expressly referenced. The verb “couple” and itsconjugated forms means to complete any type of required junction,including electrical, mechanical or fluid, to form a singular objectfrom two or more previously non-joined objects. If a first devicecouples to a second device, the connection can occur either directly orthrough a common connector. “Optionally” and its various forms meansthat the subsequently described event or circumstance may or may notoccur. The description includes instances where the event orcircumstance occurs and instances where it does not occur. “Operable”and its various forms means fit for its proper functioning and able tobe used for its intended use.

Spatial terms describe the relative position of an object or a group ofobjects relative to another object or group of objects. The spatialrelationships apply along vertical and horizontal axes. Orientation andrelational words including “uphole” and “downhole”; “above” and “below”;“up” and “down” and other like terms are for descriptive convenience andare not limiting unless otherwise indicated.

Where the Specification or the appended Claims provide a range ofvalues, it is understood that the interval encompasses each interveningvalue between the upper limit and the lower limit as well as the upperlimit and the lower limit. The invention encompasses and bounds smallerranges of the interval subject to any specific exclusion provided.

Where the Specification and appended Claims reference a methodcomprising two or more defined steps, the defined steps can be carriedout in any order or simultaneously except where the context excludesthat possibility.

FIGS. 1A-C

FIGS. 1A-C show an embodiment of the method of using an embodiment ofthe acidizing tool in a cross-sectional view of a pre-formed well boretraversing an underground formation.

FIG. 1A shows underground formation 10 containing treatment zone 15,which is a portion of underground formation 10 to be treated.Underground formation 10 and treatment zone 15 are accessible throughwell bore 20. Well bore 20 extends from the surface downward totreatment zone 15 and is defined by well bore wall 22. Treatment zone 15interfaces with well bore 20 at well bore wall 22 and extends radiallyfrom well bore 20. Treatment zone 15 has uphole bound 24, which is theuphole-most portion of treatment zone 15 accessible through well bore20, and downhole bound 26, which is the downhole-most portion oftreatment zone 15 accessible through well bore 20.

In FIG. 1A, acidizing tool 30 is introduced (arrow 32) into well bore 20such that it is positioned proximate to uphole bound 24 of treatmentzone 15. Acidizing tool 10 is introduced coupled to coiled tubing 34.Coiled tubing is operable to supply acid formulation and power from thesurface to acidizing tool 30. Acid formulation is introduced totreatment zone 15 through acid delivery system 36, which includes acidflow channels 38, which are operable to direct the acid formulation ontowell bore wall 22 in treatment zone 15.

An embodiment of the method includes where the treatment zone of theunderground formation is made of carbonate rock.

FIG. 1B shows acidizing tool 30 introducing acid formulation (jets 40)to treatment zone 15 through acid flow channels 38. Acidizing tool 30distributes acid formulation 40 radially onto well bore wall 22 fromuphole bound 24 to downhole bound 26 of treatment zone 15. Acidformulation 40 coats well bore wall 22 where distributed, which allowsthe acid from acid formulation 40 to diffuse and penetrate intotreatment zone 15, forming acid treated portion 42 of treatment zone 15.The acid penetrates into treatment zone 15 to initial acid penetrationdepth 44, which is the depth into underground formation 10 as measuredfrom well bore wall 22. FIG. 1B shows acid formulation 40 introductionwhile acidizing tool 30 is being further introduced into well bore 20.

The acid formulation includes an acid. Diluted hydrochloric and sulfuricacids are useful examples of acids solutions for the acid formulation.An embodiment of the method includes using a weak acid as the acid inthe formulation. Weak acids are acids that do not fully disassociate inthe presence of water. Acetic acid, formic acid, fluoroboric acid andethylenediaminetetraacetic acid (EDTA) are examples of useful weakacids. Weak acids are considered useful in that their reaction is notinstantaneous and total with the minerals present in the formation uponcontact but rather measured through known reaction constants, permittingapplication of ultrasound energy. An embodiment of the method includeswhere the acid has a pH value in a range of from about 2 to about 5.

The acid formulation as part of an applied gel or foam can prolongcontact with the well bore wall. The gel or foam can also reduce theamount of the acid formulation that directly contacts the well borewall, which increases the amount of unreacted acid formulation availablefor driving into the treatment zone using ultrasound energy. The foam orgel can also improve the locating of the acid formulation as the foam orgel adheres to the well bore wall proximate to where it is distributed.An embodiment of the method includes where the acid formulation is partof a gel that is operable to physically adhere to the well bore wall. Anembodiment of the method includes where the acid formulation is part ofa foam that is operable to physically adhere to the well bore wall.Pressurized gases, including nitrogen, air and carbon dioxide, areuseful for creating a foam to carry the acid formulation.

Acidizing tool 30 also includes ultrasonic transmitter 50 (showninternally). FIG. 1C shows acidizing tool 30 introducing ultrasoundenergy (arrows 52) to treatment zone using ultrasonic transmitter 50.Acidizing tool 30 transmits ultrasound energy 52 radially into treatmentzone 15 from uphole bound 24 to downhole bound 26 of treatment zone 15.Ultrasound energy 52 radiates through acid treated portion 42 oftreatment zone 15. Ultrasound energy 52 pushes the acid in acid treatedportion 42 deeper into treatment zone 15, forming ultrasonic treatedportion 54 of treated zone 15. The acid penetrates deeper into treatmentzone 15 to subsequent acid penetration depth 56, which is the depth intounderground formation 10 as measured from well bore wall 22. Subsequentacid penetration depth 56 is a greater value (that is, deeper intounderground formation 10 from well bore wall 22) than initial acidpenetration depth 44. FIG. 1C shows ultrasound energy 52 introductionwhile acidizing tool 30 is being extracted from well bore 20, whichallows acidizing tool 30 to reach the position proximate to uphole bound24.

The ultrasonic transmitter can introduce the ultrasonic energy into theunderground formation at a range of frequencies and a range ofintensities based upon the concentration, types, and amount of acidformulation used. An embodiment of the method includes introducingultrasound energy at a frequency in a range of from about 10 kiloHertz(kHz) to about 1 megaHertz (MHz). An embodiment of the method includesintroducing ultrasound energy at an intensity of sonication in a rangeof from about 1 Watt per square centimeter to about 10 (W/cm2).

The acid formulation and the ultrasound energy are directed by theacidizing tool in the same general direction to promote the dispersionof acid deep into the underground formation. An embodiment of the methodincludes where both the acid formulation and the ultrasound energy areintroduced radially from the acidizing tool. This permits total coverageof the underground formation from the well bore. An embodiment of themethod includes where both the acid formulation and the ultrasoundenergy are introduced to a focused treatment point.

The acid in the acid formulation reacts with the mineral constituents ofthe underground formation. A useful acid formulation is one where theacid has a reaction rate with the mineral constituents of theunderground formation that is lower than the rate of diffusion thoughtthe underground formation. Using a weak acid can prevent all the acidbeing consumed upon introduction to the well bore wall surface. Also,incorporating the acid formulation into a gel or a foam can also preventa majority of the acid from being consumed upon initial application tothe well bore wall. This permits maximizing the distance of diffusionthrough the underground formation, which improves the quality offormation stimulation per treatment, instead of simply acidizing thesurface of the well bore wall with the entire amount of applied acid. Anembodiment of the method includes where a significant portion of theacid does not react with the underground formation until the acid isdiffused into the underground formation by the introduction of theultrasonic energy. In an embodiment of the method, a “significantportion” means at least 50% of the acid introduced with the acidformulation. In an embodiment, a significant portion means at least 60%of the acid introduced. In an embodiment, a significant portion means atleast 70% of the acid introduced. In an embodiment, a significantportion means at least 80% of the acid introduced. In an embodiment, asignificant portion means at least 90% of the acid introduced. In anembodiment, a significant portion means at least 95% of the acidintroduced.

The difference in depth between initial acid penetration depth and thesubsequent acid penetration depth depends on several factors, includingthe intensity of sonication and frequency of the ultrasonic energy, timebetween application of the acid formulation and application ofultrasonic energy, time of exposure to ultrasonic energy, the acidcomposition, and the composition of the underground formation. Anembodiment of the method includes where the difference in depth betweenthe initial acid penetration depth and the subsequent acid penetrationdepth, as measured from the well bore wall, is at least 50% greater. Anembodiment of the method includes where the difference in depth betweenthe initial acid penetration depth and the subsequent acid penetrationdepth, as measured from the well bore wall, is in a range of from about50% to about 90% greater.

The method of treatment does not require introduction of the acidformulation in excess of the fracture gradient pressure of theunderground formation. Although potentially useful as a hydraulicfracturing or “fracking” fluid, the acid formulation useful for deepacid stimulation is operable to permit diffusion of the acid into theunderground formation through the well bore wall using fluid transportand diffusion mechanics. An embodiment of the method includesintroducing the acid formulation at a pressure less than the fracturegradient pressure value of the underground formation.

An embodiment of the method includes not introducing an externallysupplied surfactant.

FIG. 2

FIG. 2 show an embodiment of the method of using an embodiment of theacidizing tool in a cross-sectional view of a pre-formed well boretraversing an underground formation similar to FIGS. 1A-C. Acidizingtool 130 introduces acid formulation (jets 140) to treatment zone 115through acid flow channels 138. Acid flow channels 138 are located in adownhole position along acidizing tool 130. Acidizing tool 130distributes acid formulation 140 from uphole bound 124 to downhole bound126 of treatment zone 15. The acid from acid formulation 40 diffuses andpenetrates into treatment zone 115, forming acid treated portion 142.The acid penetrates into treatment zone 115 to initial acid penetrationdepth 144.

Simultaneously, acidizing tool 130 introducing ultrasound energy (arrows152) to treatment zone 115 using ultrasonic transmitter 150 (showninternal). Ultrasonic transmitter 150 is located uphole of acid flowchannels 138. Acidizing tool 130 is introduced such that for a fixedposition in treatment zone 115 well bore wall 122 is exposed to acidformulation 140 before introduced to ultrasound energy 152. Acidizing,tool 130 transmits ultrasound energy 152 from uphole bound 124 todownhole bound 126. Ultrasound energy 152 radiates through acid treatedportion 142, pushing the acid in acid treated portion 142 deeper intotreatment zone 115 to form ultrasonic treated portion 154. The acidpenetrates deeper into treatment zone 115 to subsequent acid penetrationdepth 156. Subsequent acid penetration depth 156 is greater than initialacid penetration depth 144.

FIG. 3

FIG. 3 shows a cross-sectional view of an embodiment of the acidizingtool. Acidizing tool 230 has an acid delivery system 236 with aplurality of acid flow channels 238. Acid flow channels 238 are suchthat they are operable to introduce acid formulation (jets 240) ontofocused treatment point 260. Acidizing tool 230 also has ultrasonictransmitter 250 positioned such that it is operable to introduceultrasound energy (arrows 252) onto focused treatment point 260. Theembodiment of the acidizing tool permits simultaneous introduction ofacid formulation 240 and ultrasound energy 252 onto focused treatmentpoint 260, driving acid deep into underground formation 210. Acidizingtool 230 is shown coupled to the surface with coiled tubing 234, whichsupplies acid formulation, and power conduit 262, which supplieselectrical power.

An embodiment of the method of deep acid stimulation includes where theacidizing tool both introduces the acid formulation and the ultrasonicenergy simultaneously by directing both towards a focused treatmentpoint. The focused treatment point is a point on or a short length alongthe well bore wall.

Introducing the acid formulation and the ultrasonic energysimultaneously at a focused treatment point using such an embodiment ofthe acidizing tool is useful for creating oriented fracturing within aportion of the underground formation under stress. The acidizing tool isintroduced into the well bore such that it is located proximate to thefocused treatment point. The focused treatment point is associated withthe portion of the underground formation under stress.

Simultaneous introduction of both the acid formulation and ultrasonicenergy at the focused treatment point diffuses the acid deep into theunderground formation at that location. The acid formulationintroduction does not require exceeding the fracture gradient of theportion of the underground formation under stress. The acid inside theunderground formation reacts with the formation and causes weakenedacidized spots to form.

Although shown in FIGS. 1-3 as applying the acid formulation andultrasonic energy such that the applied acid is driven into theformation in a direction perpendicular to the orientation of the wellbore, the methods of deep acid penetration and inducing stress-inducedfractures are not limited merely to angles perpendicular to the wellbore. In instances where the orientation of underground formation doesnot lend itself to deep acid penetration at a 90 degree angle relativeto the well bore, as often is the case in vertical or deviateddirectional wells applying acid formulation and ultrasonic energy intothin bands of productive formation, the ultrasonic transmitter isoperable for positioning, either remotely or pre-positioned beforeintroduction into the well bore, such that the ultrasonic energy directsthe applied acid formulation into the underground formation in anon-perpendicular angle to the orientation of the well bore. Forexample, FIG. 3 could show acidizing tool 230 having a first ultrasonictransmitter 250 positioned such that its ultrasound energy 252 isdirected at an obtuse angle relative to the orientation of the well boreand a second ultrasonic transmitter 250 oriented such that transmittedultrasound energy 252 is directed at an acute angle relative to theorientation of the well bore.

The creation of weakened acidized spots within the underground formationin conjunction with the stress in the formation causes stress-inducedfracturing of the portion of the underground formation under stress. Thestress-induced fractures are oriented fluid flow channels that not onlyfluidly connect with the well bore but also run deep into theunderground formation. In an embodiment of the method the stress-inducedfractures fluidly connect with the weaken acidized spots. Such orientedstress-induced fractures are fluid cannels useful for additionaloperations.

Introducing hydraulic fracturing fluid into the oriented stress-inducedfractures at pressures greater than the fracture gradient of theunderground formation can widen the fractures and open up previouslytight underground formations to exploitation, but in a predictable andcontrollable manner versus simply hydraulically fracturing theunderground formation.

FIG. 4

FIG. 4 shows a cross-sectional view of an embodiment of the acidizingtool. Acidizing tool 330 includes first acid delivery system 370 andfirst ultrasonic transmitter 372 coupled in series with second aciddelivery system 374 and second ultrasonic transmitter 376. Acidformulation is distributed from the surface through coiled tubing 334.Both first acid delivery system 370 and second acid delivery system 374fluidly couple to coiled tubing 334 and to one another. Power conduit362 transmits power from the surface to both first ultrasonictransmitter 372 and second ultrasonic transmitter 376, whichelectrically couple together in series. Acidizing tool 330 permitsgreater acid formulation and ultrasonic energy distribution in a singlepass through well bore 320.

Supplemental Equipment

Embodiments include many additional standard components or equipmentthat enables and makes operable the described apparatus, process, methodand system.

Operation, control and performance of portions of or entire steps of aprocess or method can occur through human interaction, pre-programmedcomputer control and response systems, or combinations thereof.

Experiment

Examples of specific embodiments facilitate a better understanding ofdeep acid stimulation method. In no way should the Examples limit ordefine the scope of the invention.

Two similar carbonate core plugs having similar physical andpermeability properties are used in order to test the effect ofultrasound waves on acid penetration depth. Both carbonate core plugsare cylindrical in form with opposing flat faces and are 35 millimeters(mm) in length from face-to-face. The first core plug has an initialpermeability value of 6 milliDarcy (mD). The second core plug has aninitial permeability value of 8 mD. Each core plug is prepared bywrapping the side of the cylinder in TEFLON (E. I. du Pont de Nemoursand Co.; Wilmington, Del.) but keeping the faces exposed.

The acid formulation for the experiment is a composition of a 5 wt %aqueous acetic acid solution. The acid formulation is maintained at 25°C. and is not stirred to maintain static conditions.

Both the first and second core plugs are partially immersed in a bathcontaining the acid formulation such that one face of the plug is influid contact with the acid formulation. The first plug is maintained inits position for two hours without any additional changes to itsenvironment. The second plug followed the same procedure except that thebath containing the acid formulation and the second plug is exposed toultrasound energy from an ultrasound source for the two-hour acidformulation exposure period. The ultrasound source directs ultrasoundenergy (at 300 kHz) at the face of the second cylinder immersed in theacid formulation.

After the two hour acid formulation immersion period, the acidpenetration distance in both the first and second plugs is determinedusing computerized tomography (CT) analysis. A CT scanner performs ascan on the two carbonate plugs at 5 mm intervals starting from thefluid-exposed face of the core plug to the non-exposed face. The CTscanner scans both core plugs before treatment to establish a baselinefor comparison. For each core plug, 7 CT “slices” along the length ofthe first and second core plugs both before and after testing help tocreate histograms that are useful in determining the effects ofultrasound energy introduction on acid penetration depth.

FIG. 5 shows the histogram depth analysis for both before and after acidformulation exposure on the first core plug. FIG. 6 shows the histogramdepth analysis for both before and after acid formulation and ultrasoundenergy exposure on the second core plug.

Histogram analysis shows that both the first and second core plugsreacted with the acetic acid in the acid formulation. A downward shiftin the CT distribution values produced by the CT analysis reflects achange in overall density of the core plug at that distance from theface exposed to the acid. The downward shift reflects that the aciddissolved mineral content from within the core plug and lowered itsoverall density. At distances where no downward shift in CT distributionoccurred indicates that the acid did not penetrate to that depth anddissolve minerals from the core plug.

The histogram analysis of the first core plug indicates that the acidpenetrated the core plug to a depth no greater than 23 mm from theexposed face. Beyond this distance, there no difference in the CTdistribution values before or after treatment of the first core plug,indicating that acid did not penetrate any further into the first coreplug.

The histogram analysis of the second core plug indicates that the acidpenetrated the core plug to a depth of almost 35 mm from the exposedface. Compared to the first core plug, the effect of introducingultrasound energy into the core plug during acid formulation treatmentincreased the acid penetration distance by at least 50%. The experimentshows that the use of ultrasound improves acid penetration depth.

We claim:
 1. A method of deep acid stimulation for a zone to be treatedin an underground formation using an acidizing tool, the methodcomprising the steps of: introducing the acidizing tool into a wellbore, the well bore operable to access the underground formation anddefined by a well bore wall, and the acidizing tool operable both tointroduce an acid formulation onto the well bore wall and to introduceultrasound energy into the underground formation; introducing an acidformulation comprising an acid onto the well bore wall at the treatmentzone such that the acid diffuses into the underground formation at thetreatment zone to an initial acid penetration depth; and introducingultrasound energy into the underground formation at the treatment zonesuch that the acid diffuses into the underground formation at thetreatment zone to a subsequent acid penetration depth, the subsequentacid penetration depth deeper in the underground formation than theinitial acid penetration depth.
 2. The method of deep acid stimulationof claim 1 where the treatment zone of the underground formationcomprises carbonate rock.
 3. The method of deep acid stimulation ofclaim 1 where the acid formulation is a gel operable to physicallyadhere to the well bore wall.
 4. The method of deep acid stimulation ofclaim 1 where the acid formulation is a foam operable to physicallyadhere to the well bore wall.
 5. The method of deep acid stimulation ofclaim 1 where the acid is a weak acid.
 6. The method of deep acidstimulation of claim 1 where the acid has a pH value in a range of fromabout 2 to about
 5. 7. The method of deep acid stimulation of claim 1where the ultrasound energy is introduced at a frequency in a range offrom about 10 KHz to about 1 MHz.
 8. The method of deep acid stimulationof claim 1 where the ultrasound energy is introduced with an intensityof sonication in a range of from about 1 and about 10 W/cm².
 9. Themethod of deep acid stimulation of claim 1 where the subsequent acidpenetration depth is at least 50% deeper than the initial acidpenetration depth.
 10. The method of deep acid stimulation of claim 1where both the acid formulation and the ultrasound energy are introducedradially from the acidizing tool.
 11. The method of deep acidstimulation of claim 1 where a significant portion of the acid does notreact with the underground formation until the acid is diffused into theunderground formation by the introduction of ultrasonic energy.
 12. Themethod of deep acid stimulation of claim 1 where the undergroundformation at the treatment zone has a fracture gradient pressure valueand the acid formulation is introduced at a pressure less than thefracture gradient pressure value.
 13. The method of deep acidstimulation of claim 1 where both the introduction of acid formulationand the introduction of ultrasonic energy occur simultaneously anddirected both towards a focused treatment point.
 14. A method of stressfracturing a portion of an underground formation, the method comprisingthe steps of: introducing the acidizing tool into a well bore such thatit is positioned proximate to a focused treatment point, the focusedtreatment point associated with a portion of the underground formationunder stress, the acidizing tool operable to direct an acid formulationand an ultrasound energy at the focused treatment point; introducingsimultaneously the acid formulation and the ultrasound energy at thefocused treatment point to diffuse acid from the acid formulation intothe portion of the underground formation under stress, the acidformulation introduced at a pressure less than the fracture gradient ofthe portion of the underground formation under stress, where the aciddiffused into the portion of underground formation under stress formsweakened acidized spots, the weakened acidized spots in combination withthe stress in the portion of the underground formation causes orientedstress-induced fractures to form that fluidly couple to the well bore.15. The method of stress fracturing of claim 14 further comprising thestep of introducing a hydraulic fracturing fluid into the orientedstress-induced fractures at a pressure greater than the fracturegradient of the portion of the underground formation under stress. 16.An acidizing tool for use in a well bore traversing through anunderground formation, the acidizing tool comprising: an acid deliverysystem operable to introduce an acid formulation onto a well bore wallof the well bore, and an ultrasonic transmitter operable to introduceultrasound energy into the underground formation.
 17. The acidizing toolof claim 16 where the acid delivery system is operable to introduce agel acid formulation.
 18. The acidizing tool of claim 16 where the aciddelivery system is operable to introduce a foamed acid formulation. 19.The acidizing tool of claim 16 where the acid delivery system isoperable to introduce acid formulation radially.
 20. The acidizing toolof claim 16 where the ultrasonic transmitter is operable to introduceultrasound energy at a frequency in a range of from about 10 KHz toabout 1 MHz.
 21. The acidizing tool of claim 16 where the ultrasonictransmitter is operable to introduce ultrasound energy with an intensityof sonication in a range of from about 1 and about 10 W/cm².
 22. Theacidizing tool of claim 16 where the acid delivery system is operable tointroduce acid formulation and the ultrasonic transmitter is operable tointroduce ultrasonic energy to a focused treatment point.